The U.S. Centers for Disease Control and Prevention said tests show some of the Mexico victims died from the same new strain of swine flu that sickened eight people in Texas and California.

Mexico put the confirmed toll at 20 dead, but 40 other fatalities were being probed, and at least 943 nationwide were sick from the suspected flu, the health department said.

Scientists said the virus combines genetic material from pigs, birds and humans in a way researchers have not seen before.

"We are very, very concerned," spokesman Thomas Abraham said.

"We have what appears to be a novel virus and it has spread from human to human," he said. "It's all hands on deck at the moment."

President Felipe Calderon cancelled a trip and met with his Cabinet to coordinate Mexico's response. The government has 500,000 flu vaccines and planned to administer them to health workers, the highest risk group.

VACCINES UNAVAILABLE

There are no vaccines available for the general public in Mexico, and authorities urged people to avoid hospitals unless they had a medical emergency, since hospitals are centers of infection.

They also said Mexicans should refrain from customary greetings such as shaking hands or kissing cheeks, and authorities at Mexico City's international airport were questioning passengers to try to prevent anybody with possible influenza from boarding airplanes and spreading the disease.

But the CDC said Americans need not avoid traveling to Mexico, as long as they take the usual precautions, such as frequent handwashing.

"We certainly have 60 deaths that we can't be sure are from the same virus, but it is probable," said Health Secretary Jose Cordova. He called it a "new, different strain ... that originally came from pigs."

Epidemiologists are particularly concerned because the only people killed so far were normally less-vulnerable young people and adults. It's possible that more vulnerable populations - infants and the aged - had been vaccinated against other strains, and that those vaccines may be providing some protection.

Dr. Anne Schuchat of the CDC said "at this point, we do not have any confirmations of swine influenza in Mexico" of the kind that sickened seven California and Texas residents. All seven recovered from symptoms that were like those of the regular flu, mostly involving fever, cough and sore throat, though some of the seven also experienced vomiting and diarrhea.

Scientists have long been concerned that a new flu virus could launch a pandemic, a worldwide spread of a killer disease. A new virus could evolve when different flu viruses infect a pig, a person or a bird, mingling their genetic material. The resulting hybrid could spread quickly because people would have no natural defenses against it.

The most notorious flu pandemic is thought to have killed at least 40 million people worldwide in 1918-19. Two other, less deadly flu pandemics struck in 1957 and 1968.

Nobody can predict when pandemics will happen. Scientists had been concerned about swine flu in 1976, for example, and some 40 million Americans were vaccinated. No flu pandemic ever appeared, but thousands of vaccinated people filed claims saying they'd suffered a paralyzing condition and other side effects from the shots.

In recent years, scientists have been particularly concerned about birds. There have been deaths from bird flu, mostly in Asia, but the virus has so far been unable to spread easily enough to touch off a pandemic.

Closing the schools across the metropolis of 20 million kept 6.1 million students home from day care centers through high schools, and thousands more were affected as colleges and universities closed down. Parents scrambled to juggle work and family concerns due to what local media said was the first citywide schools closure since Mexico City's devastating 1985 earthquake.

Authorities also advised capital residents not to go to work if they felt ill, and to wear surgical masks if they had to move through crowds. A wider shutdown - perhaps including shutting down government offices - was being considered.

"It is very likely that classes will be suspended for several days," Cordova said. "We will have to evaluate, and let's hope this doesn't happen, the need to restrict activity at workplaces."

SARS IN ASIA

Mexico's initial response in its overcrowded capital brought to mind other major outbreaks - such as when SARS hit Asia. At its peak in 2003, Beijing was the hardest-hit city in the world. Schools, cinemas and restaurants were shuttered to prevent the spread the deadly respiratory virus, and thousands of people were quarantined at home.

In March 2008, Hong Kong ordered more than a half million young students to stay home for two weeks because of a flu outbreak. It was the first such closure in Hong Kong since the outbreak of SARS, or severe acute respiratory syndrome.

Lillian Molina and other teachers at the Montessori's World preschool scrubbed down their empty classrooms with Clorox, soap and Lysol on Friday between fielding calls from worried parents. While the school has had no known cases among its students, Molina supported the government's decision to shutter classes.

"It's great they are taking precautions," she said. "I think it's a really good idea."

Still, U.S. health officials said it's not yet a reason for alarm in the United States. The five in California and two in Texas have all recovered, and testing indicates some common antiviral medications seem to work against the virus.

Schuchat of the CDC said officials believe the new strain can spread human-to-human, which is unusual for a swine flu virus. The CDC is checking people who have been in contact with the seven confirmed U.S. cases, who all became ill between late March and mid-April.